1. Field of the Invention
The present invention concerns a wavelength-division multiplex soliton signal fiber optic transmission system in which the various wavelengths of the multiplex are chosen to assure, over a given interval, relative slippage between the various channels substantially equal to a multiple of the bit time. It also concerns a method of transmission in a system of this kind.
2. Description of the Prior Art
Transmitting soliton pulses, or solitons, in the abnormal dispersion part of an optical fiber is a phenomenon known in itself. Solitons are pulse signals of sech.sup.2 shape. With this pulse shape the non-linearity in the corresponding part of the fiber compensates the dispersion of the optical signal. The transmission of solitons is modeled in a manner known in itself by the non-linear Schrodinger equation.
Various effects limit the transmission of such pulses, such as the jitter induced by the interaction of the solitons with noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, Optical Letters, vol. 11 No. 10 pages 665-667. This effect, known as the Gordon-Haus effect, imposes a theoretical limit on soliton transmission quality or bit rate.
Synchronous modulation of soliton signals by means of semiconductor modulators can be used to exceed this limit. This technique intrinsically limits the bit rate of the soliton link because of the upper limit of the pass-band of the semiconductor modulators. Sliding-frequency guiding filter systems controlling the jitter of the transmitted solitons have also been proposed, for example in EP-A-0 576 208.
Using the Kerr effect in synchronous amplitude or phase modulators to regenerate the signal on the line has also been proposed. These techniques necessitate high-frequency components, whence the benefit of an all-optical approach. A non-linear optical loop mirror (NOLM) can be used for the amplitude modulation and the fiber itself for the phase modulation. A presentation by S. Bigo, P. Brindel and O. Leclerc to the "Journees nationales de l'Optique guidee", Oct. 30, 1996, Nice (France), described regeneration of a soliton signal by all-optical phase modulation. An optical clock is superposed on the soliton signal which imposes a non-linear phase-shift on the pulses of the soliton signal, copropagating with them in an optical fiber. A section of the fiber is selected to minimize the effects of slippage between the soliton signal and the optical clock. Reference may usefully be had to T. Widdowson et al, Soliton shepherding all optical active soliton control over global distances, IEE Electron. Letters, vol. 30 No. 12 p. 990 (1994).
Using wavelength-division multiplexing to increase the bit rate of soliton signal fiber optic transmission systems has also been proposed. In this case it is considered advantageous to use Fabry Perot type sliding-frequency guiding filters which are entirely compatible with wavelength-division multiplexed signals. On the other hand, problems can occur if synchronous modulators are used to regenerate wavelength-division multiplexed soliton signals because of the group velocity difference between the signals on the various channels.
An article by E. Desurvire, 0. Leclerc and O. Audouin, Optics Letters, vol. 21, No. 14 describes a wavelength allocation scheme compatible with the use of synchronous modulators. The article proposes to allocate wavelengths to the various channels of the multiplex so that, for given distances between repeaters, the signals of the various channels are synchronized on reaching the repeaters. This enables in-line synchronous modulation of all channels, at given intervals, by means of discrete modulators. The article specifies a maximal value of 5.10.sup.-3 for the relative variation in position of the repeaters, which assures that the relative variation in the bit time remains below 5% for five channels. This technique cannot be applied directly to all-optical modulators utilizing the Kerr effect or to distributed modulators because the dispersion in the fiber causes slippage between the various channels and the clock assuring the modulation.
In a dissertation entitled Traitement de signal tout-optique pour la transmission a tres haut debit de solitons par fibre optique (University of Besancon, 1996) S. Bigo has shown that an all-optical modulator utilizing the Kerr effect, such as an NOLM or a fiber, can be considered as a discrete sinusoidal modulator synchronized with the stream of solitons, despite the slippage ("walkoff ") due to chromatic dispersion and losses, if the clock is sinusoidal and the temporal offset between the signal to be modulated and the clock is adjusted.
The present invention proposes an original and simple solution to the problem of all-optical modulation of wavelength-division multiplexed soliton signals. It enables existing solutions (amplitude of phase distributed optical modulation) to be applied to wavelength-division multiplexed soliton signals despite the slippage caused by group velocity differences.